Because mandatory individual insurance is so central to the bill, Judge Vinson struck down the entire act.

The case is likely to end up in the Supreme Court.

Judge Vinson is the fourth federal judge to rule on the constitutionality of the reform bill.

Two other judges have upheld the law but one judge in Virginia also struck down the individual mandate.

Judge Vinson's 78-page ruling was broader than that of his counterpart in Virginia, but stopped short of ordering the federal government to stop implementing the law.

"This is obviously a very difficult task. Regardless of how laudable its attempts may have been to accomplish these goals in passing the Act, Congress must operate within the bounds established by the Constitution," Judge Vinson wrote.

The US Department of Justice has said it will appeal against the ruling.

House Republicans recently voted to repeal the healthcare law, but the repeal is not expected to pass in the Senate.

Regardless, President Barack Obama has vowed to veto a repeal should such a bill make it to his desk.

The president's landmark legislation, passed last March, would provide coverage to more than 30 million uninsured people.

The law would also expand a Medicaid programme for the poor and provide tax credits to make premiums more affordable for the middle class.